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WiFi Antennas - Ground Plane?


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#1 Master_Scythe

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Posted 07 November 2017 - 10:39 AM

I was thinking about WiFi the other day.

Out 'straight' WiFi antennas are effectively WHIP antennas like found on a vehicle or any other radio correct?

 

As such, that makes the 'pin' in the middle our actual antenna, and the metal screw part, the ground.

Yes?

 

Now, I learned all about ground plane when I was doing CB radio stuff; and I'm assuming this applies to all types of whip antenna\frequency?

 

 

As such;

if I have enough room in something, is it worthwhile extending the ground plane?

 

Take a wifi router;

They have an RP-SMA connector exposed, if I was to say, cut an aluminium can and slide it over the threaded section (making tight contact), before I screwed the antenna on, am I getting a significantly improved ground plane, and therefore distance\noise rejection?

 

Or is there something about 2.4Ghz in general that doesn't work that way?

 

I'm asking for two separate projects.

 

First, I'm trying to keep wifi TX as low as possible in my house, as I find it keeps me awake some how; Dont ask, I'm not a 'new age guy', all I know is that wifi router on = less sleep; even in a room with no tech.

 

Second, I'm getting really into RC stuff, which uses a 2.4ghz frequency, and I was hoping to improve my range after doing a very common 'wifi antenna mod' on my transmitter.


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#2 Rybags

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Posted 07 November 2017 - 12:18 PM

A fat ground plane helps suppress RFI both in and out.  I suspect that having the antenna as part of or encased in the ground plane would inhibit the transmission process.

A lot of those tricks you see with CDs etc being appended to the antenna tend to just make the signal more directional, so improving it in some places but making it much worse in others.



#3 Master_Scythe

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Posted 07 November 2017 - 02:16 PM

A fat ground plane helps suppress RFI both in and out.  I suspect that having the antenna as part of or encased in the ground plane would inhibit the transmission process.

A lot of those tricks you see with CDs etc being appended to the antenna tend to just make the signal more directional, so improving it in some places but making it much worse in others.

 

Yeah, good point.

 

Though the old CD trick was more about satellite dishing the signal.

 

Difference here would be that the outside of the coax (is it called a ground?) would be extended by the metal.

 

So normally, you'd only have the threaded metal body itself as the ground plane.

 

But if you whacked part of a drink can, or even applied a very wide washer, before screwing the antenna down; now you actually have SOME form of ground plane.

 

Will it help? No? Yes?

I'm finding it hard to figure this out.

 

Or, perhaps the ground plane is where the 'coil' in the wire is? perhaps I'd need an antenna that looked somewhat like    ---------|--------- rather than ----------~-------- that it does now.

 

 

FNO98RTGOW48ZAT.RECT2100.jpg

 

Fruck I miss redhatter sometimes :P

He'd be all over this shiz.


Edited by Master_Scythe, 07 November 2017 - 02:18 PM.

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#4 Rybags

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Posted 07 November 2017 - 03:37 PM

Not even sure if they share the Gnd between chassis and signal... that might be the first thing worth investigating.



#5 Master_Scythe

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Posted 08 November 2017 - 09:12 AM

Not even sure if they share the Gnd between chassis and signal... that might be the first thing worth investigating.

 

So I'm going to steal a post from rcgroups, done by RIbble.

 

 

The metal sleeve (tube) is really the other half of a "coaxial dipole". The remaining whip is 1/4 wave and the metal sleeve is 1/4 wave, the whip is the coax center conductor extended and the metal sleeve is connected to the coaxial cable shield at the dipole feedpoint. A quite good antenna.

"Coaxial dipole" name comes from the common quick antenna build where you take a length of coaxial cable, remove 1/4 wavelength of the outer jacket, then fold the 1/4 wavelength shield back on the coaxial cable. End up with a 1/4 wave whip and 1/4 wave going in the other direction, making a simple 1/2 wave dipole. Easier to picture this as a vertical whip with the other half of the dipole (shield) going back towards the transmitter, and the coaxial cable feedline with insulated jacket is under the shield to isolate the coaxial cable from the bottom half (shield) of the dipole.

A simple 1/4 wave whip (without the metal sleeve) is called a "groundplane" antenna, where the whip works against the "ground" or metal common foil on the printed circuit board on our transmitters or receivers. In general, a 1/2 wave dipole has twice the power gain of a 1/4 wave groundplane antenna.

A 1/2 wave dipole is usually two 1/4 wave wires going in opposite directions with the coaxial cable connected in the middle. Works great when horizontal, but when vertical one half of the dipole is parallel to the coaxial cable and the directional pattern is messed up badly. By putting the coaxial cable inside the lower half of a vertical dipole (metal sleeve), the radiation pattern is not messed up (is "decoupled" from the coaxial cable).. Also a little problem in that the dipole is a balanced antenna being fed from an unbalanced coaxial cable feedline.

A proper 1/4 wave groundplane antenna is an unbalanced antenna being fed by an unbalanced coaxial cable. "Unbalanced" means that the currents are unequal. TV twinlead is an example of a balanced feedline, usually connected to a balanced dipole.

 

 

So, if I'm right, that metal sleeve at the bottom is likely the other half of a 1/2 wave dipole. and NOT a groundplane antenna like I thought.

 

Though, this does mean making a full wave antenna would be easy enough....

 

1. Buy Coax

 

2. Attach SMA connector

 

3. Strip 62.5mm of wire

 

4. Fold the shielding back on itself, and add a copper tube, ALSO 62.5mm

 

That should be a bloody cheap coax dipole at a full wave length, and I'd imagine better than anything we could buy.....

 

Also it means we can remove the Dilectric layer, if we know the antennas are safe in our own homes.


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"I don't care what race you are, not one f*cking bit, if you want to be seen as a good people, you go in there and you f*ck up the people who (unofficially) represent you in a negative light!"


#6 stadl

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Posted 08 November 2017 - 10:23 AM

Sorry, I don't understand the issue. You want to reduce RF emissions of the router, but you are endeavouring to adjust the ground plane or antenna with the intention of increasing RF emissions.

 

Making the antenna more efficient by improving the ground plane or using a better antenna will do one of 2 things.

1. make the antenna more efficient at converting between electrical energy to RF energy - so you can either

    a. radiate more RF for the same transmitter power, thus getting bigger range (at the same time being more sensitive to receive from a larger range)

    b. wind down the transmitter power to achieve the same level of radiated power - it can extend battery life for mobile devices but for a mains powered device, at home wifi levels, you are unlikely to detect it on a power bill.

2. Change the radiation pattern, offering directional gain (in some 3D way) that makes it effectively more efficient when communicating with devices in the beam - see #1, and less efficient for everywhere else

 

If you want less RF emissions, simply drop the TX power of the router.


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#7 Master_Scythe

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Posted 08 November 2017 - 11:23 AM

Sorry, I don't understand the issue. You want to reduce RF emissions of the router, but you are endeavouring to adjust the ground plane or antenna with the intention of increasing RF emissions.

 

Making the antenna more efficient by improving the ground plane or using a better antenna will do one of 2 things.

1. make the antenna more efficient at converting between electrical energy to RF energy - so you can either

    a. radiate more RF for the same transmitter power, thus getting bigger range (at the same time being more sensitive to receive from a larger range)

    b. wind down the transmitter power to achieve the same level of radiated power - it can extend battery life for mobile devices but for a mains powered device, at home wifi levels, you are unlikely to detect it on a power bill.

2. Change the radiation pattern, offering directional gain (in some 3D way) that makes it effectively more efficient when communicating with devices in the beam - see #1, and less efficient for everywhere else

 

If you want less RF emissions, simply drop the TX power of the router.

 

Sorry, my question was indeed rather double ended.

My router is on the minimum TX, so I'm trying to make the best antenna I can using the weakest TX power.

Mainly for the receive side, as I often see 3 of 5bars of signal, but get no reply. So the weak point seems to be the routers RX from the smaller devices.

 

 

Regardless; the OTHER hobby involves the best of it all; RC Transmitters.

Most power output, and the best antenna possible.

As such, I'm learning about these 'Coax Dipoles' and I think I'll start by making a "Full wave" dipole, consisting of a 1/2 wave exposed wire, and a 1/2wave ground.

 

Thoughts?


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"I don't care what race you are, not one f*cking bit, if you want to be seen as a good people, you go in there and you f*ck up the people who (unofficially) represent you in a negative light!"


#8 stadl

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Posted 08 November 2017 - 04:59 PM

ahh ok. Improving receive sensitivity will help when you have an uneven link like that.

 

Just remember that the quickest thing to try is to get the polarisation right - If the AP antenna is vertical, and the device antenna is horizontal (or vice-versa) you lose 3dB (half) - something that makes mobile network engineers laugh when people point their rear windshield mobile/radio towards the back of the car car because they think it looks cooler :)

 

I'm very rusty on coax dipoles, but they are an affordable and easily home brewed antenna. I haven't tried them up at 2.4GHz, not sure what coax would work best. RG58 and RG8 series coax are common at lower frequencies, because they are common, cheap, and have enough strength for the lengths required.

 

At 2.4 GHz, the antenna size will be small and measurements will probably need to be quite accurate. Perhaps RG178 or 316 would work - also good because it's common to find pre-made SMA terminated tails that can be cut up to make one - ebay is full of them for $few or you may have them lying around if you have been using random wifi gear for a while :)

 

Another option is directional antennas - you can make a 2.4GHz Yagi antenna using a PCB - probably can etch it at home for not much money - even making one by glueing wires to a flat plastic surface could work. And feel free to google cantenna (although pringles have changed the size of their packaging - haven't checked if they still work :P )


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#9 merlin13

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Posted 11 November 2017 - 07:41 PM

Used to do military RF and industrial radio control/comms for a few years so I'll add in some Fun Facts. Apologies for the long diatribe.
 
 - Cross-polarising dipoles (ie having one vertical and the other 90 degrees horizontal but still perpendicular to the other) will nominally drop your RF levels by 30dB. Useful if/when your RF levels are too high (also works for Yagi antennas as well but here's other tricks 'n traps with that style of stick that can be exploited, but not here...).
 
 - Having a dipole point directly up along it's length at your destination/originating signal source will drop it metric shitloads, as doing that you'll end up trying to use the worse possible portion of the antenna's doughnut-shaped propagation pattern - you'll actually be relying on reflected signal pathing from objects/structures within the RF pathing fields. Useful if the levels are stoopidly too high. Also useful to help directionalise the RF energy when using dipoles.
 
 - Dipoles work exactly the same pointing straight up or straight down. The only differences are in the actual RF signal pathing and placement of any obstacles and nearby RF reflective/absorbitive   surfaces etc when you place the dipole in what 99.999% of the public consider to be such an unnatural orientation. For example - if you want to fit a stick on your house, then putting aside the drop in vertical height (and potential impact on Line Of Sight, which all antennas love) and the wall then behind it, there's nothing wrong installing a dipole up under the eaves of the roof pointing downwards.
 
 - RF can be one of the most insidious forms of energy known to Man, ask anyone who's been involved with shield rooms for example. If you've got issues there with sensitivity to RF then look at repositioning the antennas (thence altering propagation patterning/RF pathing) such as the help minimise any effect you feel - for example put an antenna under the desk, or up on the top of the cupboard well above your head height. Even up into the roof cavity.
 
 - Oh, and be aware home-rolling coax leads can be problematic, as it's possible to cut coax lengths such as to act as ideal RF paths, dead shorts or even open circuits, all based on actual conductor lengths vs RF frequency. 
 
 
Back to one of your original problems.
 
 - If you want to drop RF transmissions levels even further than the minimum Tx setting in your router, have a go at adding a suitable RF attenuator onto the coax lead. But be aware this will affect RF levels in both directions, Tx and Rx.
 
 - A reeeeally sneaky trick along these lines is to not screw the connector all the way home, leaving a small but finite air gap 'tween the coax and antenna centre pins. Be aware though, apart from being somewhat Trial and Error With Associated Fiddling this can also become somewhat icky for consistent/long-term stability if you're reeeeeally anal about the numbers - temperature variations in the metals involved for example can make quite significant variations in this air-gap attenuation over time.
 
 - If you simply (!) want to drop Tx levels but keeps the Rx sensitivity levels up then you can start looking at either a router/WiFi device with separate fixed or configurable Tx and Rx ports, then putting a small or lower-gain antenna on the Tx and a higher gain antenna on the Rx one.
 
 - Or start to look at fiddling with Diplexers. and different gains of antennas for the Tx and Rx ports.
 
 
Now noting an important item in your last post - "...often see 3 of 5bars of signal, but get no reply..."
 
Rule 1 - when connectors are involved, always check that the centre pin isn't bent or been pushed back, and always check for kinks or sharp bends in any coax as well. Either of these will screw up the RF levels/sensitivities, only needs one at the right end to give you grief.
 
Rule 2 - check the RF numbers/levels at BOTH ends of the radio paths and compare each end. Ideally they should be equal for both devices.
 
Rule 3 - ideally swap the two devices end-for-end on the troublesome radio path as well, then recheck the numbers - they should follow the devices around. If they don't then you might have external RF interference affecting Rx sensitivity at one end of the path. 
 
Rule 4 - do not assume the devices (both routers/wifi cards and even the antennas themselves) have the same Rx sensitivities, nor that the configured Tx levels/numbers and listed Gain numbers (on antennas) are actually the truth, until proven otherwise. Trust Me On This - I've encountered oodles of both Yum Cha and Name/Brand WiFi antennas and various radio equipment at various frequencies (from near DC to microwave) that under real testing turn out to be basically stuffed/misleading...
 
If you're experiencing acceptable Rx levels at one end but not the other then a simple fix can sometimes be add a higher gain antenna at the lower Rx level end. 

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#10 chrisg

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Posted 17 November 2017 - 03:55 PM

:)

 

We messed around with those coax "antennas" years ago, to our great surprise with short lengths RG-59 was as good as anything else - it shouldn't be but it was. We were using mil-spec which you can have a bit of a hunt for these days but most everyone has some -59 hanging around - worth a try :)

 

Been a lot of years tho, I've been using yagis in the main for anything needing a bit of range but inside the smallish houses I've lived in for several years the built-in wi-fi does all that I need :)

 

Have fun, oh, and Merlin really does know his stuff in this and a number of other areas :)

 

Cheers


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#11 merlin13

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Posted 17 November 2017 - 05:28 PM

Aww shucks, Biggles. Now I'm blushing...


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#12 chrisg

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Posted 17 November 2017 - 09:59 PM

Aww shucks, Biggles. Now I'm blushing...

:)

 

Credit where credit is due - just stay out of my cockpit :)

 

Cheers


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#13 Rybags

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Posted 17 November 2017 - 10:09 PM

As it happens, meanwhile back at the ranch (homepage) there's an article on homebrewed alfoil mods...

 

https://www.pcauthor...-kitchen-477322

 

Researchers at Dartmouth College found that the use of aluminium foil does indeed increase the range of Wi-Fi connections and can also improve its security.


#14 merlin13

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Posted 18 November 2017 - 11:53 AM

 

Aww shucks, Biggles. Now I'm blushing...

:)

 

Credit where credit is due - just stay out of my cockpit :)

 

Cheers

 

Will cheerfully stay riiiight away from your 'pit...


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#15 chrisg

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Posted 18 November 2017 - 01:51 PM

 

 

Aww shucks, Biggles. Now I'm blushing...

:)

 

Credit where credit is due - just stay out of my cockpit :)

 

Cheers

 

Will cheerfully stay riiiight away from your 'pit...

 

:)

 

I seem to have my flying bug back, flown some wild aircraft the past few weeks - real challenge is not letting the wife know :) Had a ball though here at what I suspect is the twilight of my career - time to pass along the skills :)

 

Cheers


"Specialisation is for Insects" RAH




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